7.3 doc
Somewhere in chapter 9 on openssh, I believe there should be a note explaining
that if a fresh install is performed, new keys will be generated for the server.
If a user (sysadmin) want to keep the identity of the system the same as an old
identity so that authorized keys don't have to up updated on the other system,
they should copy the /etc/ssh/ssh_host*key* files from the old system to the new
one.